Plants are often used as a source for starch, which can be used to produce ethanol and other products. Plant starches are generally in a granular form, which is insoluble in water. Starch is initially collected from plant grains using either a wet milling, a dry milling or a dry grind process.
To produce ethanol, starch containing fractions derived from wet milling or ground grain from dry grinding are further hydrolyzed into fermentable sugars which are then fermented to make ethanol. Several plant starch processing methods exist including a raw starch process, which involves little to no heating of the milled plant material being processed; or higher temperature hydrolysis of starch frequently referred to as “liquefaction”. In either of these methods for breaking down starch derived from plants, the conventional process involves the addition of enzymes, frequently liquid enzymes, to the milled plant starch in a slurry tank.
Liquefaction methods often involve a starch gelatinization process, wherein aqueous starch slurry is heated so that the granular starch in the slurry swells and bursts, dispersing starch molecules into the solution. During the gelatinization process, there is a dramatic increase in viscosity. To enable handling during the remaining process steps, the starch must be thinned or “liquefied”. This reduction in viscosity can be accomplished by enzymatic degradation in a process referred to as liquefaction. During liquefaction, the long-chained starch molecules are degraded into smaller branched and linear chains of glucose units (dextrins) by an enzyme, such as alpha-amylase (i.e., α-amylase).
A conventional enzymatic liquefaction process comprises a three-step hot slurry process. The slurry is heated to between 80-85 degrees C. to initiate gelatinization and α-amylase is added to initiate liquefaction. The slurry is jet-cooked at temperatures between 105 and 125 degrees C. to complete gelatinization of the slurry, cooled to 60-95 degrees C. (e.g., between 90-95 degrees C.), and, usually, additional α-amylase is added to finalize hydrolysis during a secondary liquefaction step. This three step process is employed in order to break down as much of the plant starch as possible. A process that avoids the use of a jet cooker or a secondary liquefaction would be advantageous to ethanol production as this would save considerable time, effort, and costs associated with heating, cooling and transferring the slurry between tanks.
Liquefaction results in the generation of dextrins as the starch is hydrolyzed. The dextrins can be broken down further during saccharification, to produce low molecular weight sugars that can be metabolized by yeast. The saccharification hydrolysis is typically accomplished using glucoamylases and/or other enzymes such as α-glucosidases and/or acid α-amylases. A full saccharification step typically lasts up to 72 hours. However, it is also common to perform only a pre-saccharification step of about 40 to 90 minutes at a temperature above 50 degrees C., followed by a complete saccharification during fermentation in a process known as simultaneous saccharification and fermentation (SSF).
Prior to entering the fermentation tank, the slurry must be cooled to about ambient temperature. The slurry is typically pumped through a heat exchanger to cool the slurry. It is important that the slurry remain in a relatively fluid form during this process. As the slurry thickens due to cooling, it places added pressure on the heat exchanger. A process improvement that avoids excessive thickening of the post liquefaction slurry when cooled to ambient temperature is an advantage to the ethanol producer.
Fermentation can be performed using yeast, e.g., a Saccharomyces spp. Typical SSF times range from 40 to 60 hours. After SSF, ethanol is recovered by distillation. The residual solids and liquids can be dried to make the fermentation co-product dried distillers grains (DDG) and dried distillers grains and solubles (DDGS). A portion of the liquid streams from the distillation (referred to as backset or stillage) can be recycled back to the process.
The natural pH of milled corn grain is approximately pH 5.8 to approximately pH 6.0; however when combined with recycled process water (backset) in the slurry tank, the pH is approximately pH 4.8 at 30 to 33% dry solids. The pH is typically adjusted to approximately 5.8 before the liquefaction step in order to be compatible with the biochemical properties of commercial thermostable amylases. An large dosage of the enzyme would be required at pH 4.8. After the jet cooking and secondary liquefaction, the pH must be decreased to pH 4.8 for simultaneous saccharification and fermentation (SSF), saccharification and subsequent fermentation, or fermentation without saccharification.
Given the increasing importance of ethanol as a fuel, improvement to the process is desirable.